It has been known for long to manufacture blanks by sintering from powdered metal. Their manufacture requires a press, an eccentric or hydraulic press, which has a die-plate and at least one lower ram and an upper ram. The lower ram closes the bore of the die-plate from bottom while it is being filled with the appropriate powder. After the so-called precision proportioning during which the upper ram travels upwards to a predetermined position and powder located beyond the die-bore is stripped the upper ram is lowered and the lower ram is moved up and the powder mass is compacted between the rams. While undergoing compaction, the powder particles which have a grain size of about 0.1-500 μm are “entangled” with each other so that a structure arises which is stable in shape, may be readily removed from the press, and may be subjected to a further processing operation, particularly to sub-sequent sintering.
During sintering, dimensional variations will occur because of shrinkage phenomena. Shrinkage is a material constant, on one hand, but is also dependent on the density of the compact, on the other. The crucial factor in filling, therefore, is that the compact be of a density as uniform as possible across its extension. If this requirement is not met it will possibly be necessary to rework the blank. For instance, if a milling cutter blade is manufactured by such a sintering process and it lacks the desired dimensional accuracy after being sintered it needs regrinding. This, however, increases the expenditure in manufacture and fully or partly cancels the benefit of sintering which involves less expenditure as such.
Particularly problematic are components of a complex shape, e.g. also reversible cutting blades including specific clamping grooves and land-and-groove chip breakers.
The die-plate is usually filled by means of a so-called charging shoe connected to a powder source. The charging shoe moves on the die-plate surface surrounding the die-bore and is filled with powder. The powder falls into the die-bore while the shoe is moving across the bore. The shoe is subsequently shifted back to the initial position. During both the filling stroke and return stroke, a one-sided compaction occurs at the border of the die-bore, i.e. in the regions that are located on the axis along which the charging shoe is shifted.
The company document “Osterwalder Verfahrenstechnologie” of the Osterwalder AG, Industriering 4, CH-3250 Lyss, has made it known to generate different speeds during the filling procedure using the charging shoe while additionally vibrating the die-plate at the same time. This company document has further made it known to carry out a motion of the die-plate, which is precisely defined and in synchronism with the filling axis, while the charging shoe is travelling back.
DE 199 03 417 has also made it known to move the charging shoe, during the filling procedure, beyond the die-bore in at least two further directions different from the first direction. The compaction which comes into being on the associated wall of the die-bore is distributed in an approximately uniform way here across the circumference of the die-bore.
From “Mikroprozessor gesteuertes hydraulisches Pressen in der Pulvermetallurgie aus Werkstatt und Betrieb” of 1986/6, pages 80ff, it has become known to fill in the powder by means of programmable speeds and times, vibrating motions or oscillations during the filling procedure. Then, the compression procedure is effected according to a typical procedural chart.
As was mentioned previously the aim in manufacturing cemented-carbide compacts is to achieve a density as equal as possible for all compacts, particularly in their cutting areas. This aim is achieved only incompletely by the known measures described.
Therefore, it is the object of the invention to provide a process for compressing metallic powder into a compact in a powder press wherein the aim of imparting the same density to all compacts across their extension is achieved even better.